Abrasive bonds are used in the manufacture of abrasive discs or grinding wheels intended for various grinding applications. The discs are made to specifications which suit them for specific applications, such as dry or wet grinding, light or heavy stock removal, and a range of workpiece materials. Disc specifications typically include a description of abrasive grain types and sizes used, disc grades (strength of bond), disc structure (density of abrasive grain) , disc bond type (resinoids, cementitious, ceramic, other) , and disc configuration. This last refers to characteristics of the face (surface) of the disc, which is varied according to application needs, such as cutting action, temperature control requirements, size and type of workpiece, and desired finish.
For applications requiring a fast, cool cutting action, disc faces are often configured with a plurality of holes punched through the thickness of the disc perpendicular to the plane of the face, which serve to improve coolant distribution during use; these wheels are referred to in the art as "corrugated" wheels, and the holes are referred to as "corrugations." Combination configurations for special applications include corrugations with radial grooves and combinations of corrugated and smooth surfaces on the same disc face. Corrugations in small, medium, large and extra large sizes are in use. Corrugated discs and grinding wheels of the type useful in the practice of the present invention are illustrated and discussed in "High Performance Abrasive Products" (1997), a publication of Landis-Gardner Co., 20 E. Sixth Street, Waynesboro, Pa., USA, incorporated herein by reference.
It has previously been proposed to modify the properties of corrugated discs by plugging the corrugations. This concept has several advantages in theory: the plugs provide an interrupted cutting action which is useful for larger surface area parts or heat sensitive parts where constant contact with the wheel face can generate too much heat and cause warping or burning of the part; the wheel costs less to produce than a solid wheel, as more expensive abrasive materials such as diamond, CBN, or sol gel grain abrasives can be limited to the plug elements with little, if any, change in grinding result; and spalling caused by heat concentration at the center of wheels such as nut-inserted discs may be reduced.
In practice, however, it has not been possible to effectively plug corrugations in water-sensitive bonds with desirable aqueous-based plug materials. Owing to the size of the corrugations ("small" corrugations are typically about 1/4"-3/8" in diameter and 3" deep), the plug material must initially have very low viscosity to permit the corrugations to be readily filled without significant air entrapment. This means that typical aqueous-based settable compositions for use as corrugation fill material must initially contain an excess of water for lowering the viscosity of these compositions to a useful range for filling.
Unfortunately, this approach is not feasible for plugging corrugated cementitious abrasive bonds such as magnesium oxychloride bonds ("oxychloride bonds"). It is well known that these bonds are highly sensitive to water. The excess water in these aqueous fill compositions inevitably seeps or is drawn into the wheel matrix while the compositions set up, irrespective of the specific material employed. This phenomenon results in a volume change in the bond matrix which degrades the bond, frequently causing radial cracks or other imperfections which render the wheel unusable. Accordingly, a different method for plugging corrugations in cement-bonded abrasive grinding wheels with aqueous-based material has been needed.